Electric motors for converting electrical energy into rotary motion are highly efficient devices, particularly in the larger sizes, but much of this efficiency is lost in the process of converting this rotary motion into linear or reciprocating motion as is necessary in some machines, such as reciprocating pumps and positioning equipment.
As used herein, the term linear pump or motor means a device having a piston that translates along its axis rather than rotating about its axis; the axis along which the piston travels may be a straight line, or curved line, or a combination of both. The two principal obstacles to the development of an efficient linear motor have been the difficulty of establishing tight flux linkages between the stator and armature, and the complexity of the control circuitry needed to drive the motor.
Previous efforts have incorporated permanent magnets with poles facing axially toward the ends of the cylinders. In order to establish adequate coupling in this configuration pole pieces have sometimes been used in the ends of the working cylinder, in which case stroke length is severely limited as shown in U.S. Pat. No. 2,701,331 to Holst and U.S. Pat. Nos. 3,754,154, 3,846,682 and 3,884,125 to Massie. The concept of using a plurality of coils has been investigated as shown in U.S. Pat. No. 4,541,787 to Delong, but at the cost of good magnetic coupling between drive coils and piston. These designs also called for relatively thick metallic cylinders, further reducing magnetic coupling and introducing the additional complication of induced eddy currents in the cylinder caused by the electrical current in the drive coils.
Rotating synchronous electrical machinery has an advantage over linear equipment in that some slippage is allowable. If magnetic coupling between stator and armature poles is lost in a rotating magnetic field such as is encountered in ordinary induction motors it is quickly reestablished with the following armature pole, and the result is only a minor loss of efficiency. In a linear motor, loss of magnetic coupling results in erratic behavior or complete motor stoppage. In the prior art, drive coil currents were controlled by passive circuitry such as mechanical switching or electronic oscillators, and currents were applied without reference to the actual position of the piston. Such a motor would operate efficiently only in a very narrow range of conditions, and would not operate at all if momentarily overloaded. In the present invention this difficulty has been overcome by the use of electronic circuitry capable of sensing piston or armature position and supplying current pulses so as to eliminate slippage.